Microstrip antennas have interesting features for aerospace applications, in particular their low weight and thin profile. By combining patches into arrays, their inherently low directivity can be overcome. They can easily be mounted on flat or even gently curved surfaces. Printed antennas, more commonly referred to as patches using microstrip technology, can use square, circular, elliptical or even more complex shapes as radiating elements. The shape selection is dependent upon the parameters that are to be optimized, such as: bandwidth, side lobes, or cross-polarization. Since patches only radiate close to their resonant frequencies, their main dimension is about one-half-wavelength. The drawback of low directivity may be overcome by grouping a plurality of patches to form an array. A new technique is described here that permits the phasing of each patch in an array to be adjusted so as to form a main beam in an arbitrary direction.
I. Prior Art
There are several patents that disclose various reflectarrays that have been theoretically analyzed. Further, several other reflectarrays that perform at lower frequencies have been demonstrated.
II. Brief Discussion of the Prior Art
The referenced prior art that follows, show ferroelectric films for phase shifting and microstrip patch elements, as well as, circularly polarized microstrip antennas. In particular:
U.S. Pat. No. 5,589,845, granted Dec. 11, 1996, to R. M. Yandrofski, et al., discloses a tunable small patch antenna and phase shifters utilizing ferroelectric thin films and superconducting films. Also disclosed is the application of thin ferroelectric and superconducting films to tunable capacitors for microwave circuits. There is no reference to the use of these films for reflectarray antenna applications and no indication of how to design and implement phase shifting devices that are particularly suitable for such antennas. References were only to microstrip and coplanar waveguide delay lines, where recognition to the performance advantages of the couple line phase shifter design was extant. Yandrofski, et al, only alludes to phased array antennas in a generic sense. An important distinction between the phased array antenna technology understood by Yandrofski, et al, and the novel reflectarray antenna herein disclosed is that the former requires a beamforming manifold. The delay lines referred to would ostensibly be incorporated within this manifold to provide beam steering. Further disclosed is an inaccurate generalization that microwave performance of such devices is limited mostly by the loss tangent of the ferroelectric dielectric layers.
U.S. Pat. No. 5,589,842, granted Dec. 31, 1996, to J. J. H. Wang, et al, discloses a compact broadband microstrip antenna of patch elements with a ferromagnetic substrate. Wang, et al, further discloses a compact, broadband antenna subsystem, of 300% bandwidth, that can generate particular modes of radiation. This antenna may be loaded with ferromagnetic material to effect the moding and reduce the size. The tuning, as disclosed, relates to frequency agility only. Consequently, this antenna design does not rely on ferroelectric technology for operation; wherein the use of phase shifters to effect beam scanning is not disclosed.
U.S. Pat. No. 5,561,407, granted Oct. 1, 1996, to T. K. Koscica, et al., discloses a single substrate ferroelectric phase shifter. Further disclosed is a multiple section microstrip line woven over a ferroelectric layer. The circuit, as disclosed by Koscica, et al., provides a limited predetermined and discrete amount of phase shift. As a result, this type of phase shifter could result in somewhat degraded phased array performance because of the granularity in the scanned antenna pattern.
U.S. Pat. No. 5,543,809, granted Aug. 6, 1996, to C. K. Profera, Jr., et al., discloses a planar reflectarray antenna for satellite communications. Incorporated by Profera, is a dual-polarized reradiating antenna subsystem. This design permits frequency reuse because of orthogonally polarized dipole antenna elements. What is disclosed is detailed as how to control the phasing of each element of the array in order to form a cophasal (collimated) wave front. This design is intended for use in a fixed beam system, without including the use of ferroelectric phase shifters or any other type of phase shifting device.
U.S. Pat. No. 5,472,935, granted Dec. 5, 1995, to R. M. Yandrofski, et al., discloses a tunable microwave ferroelectric patch antenna and phase shifter. This design details tunable, low-loss microwave devices that are based on thin ferroelectric and superconducting films, but no specific phase shifter or phased array antenna designs are provided.
U.S. Pat. No. 5,434,581, granted Jul. 18, 1995, to G. Raguenet, et al., discloses a broadband antenna of subarray patches on a dielectric substrate. Further disclosed is a technique for enhancing the bandwidth of a microstrip patch antenna, or an array of such patches. This technique involves encasing the patch in a metal cavity with various but specific geometry.
U.S. Pat. No. 5,382,959, granted Jan. 17, 1995, to T. A. Pett, et al., discloses a high performance broadband circular polarization and microstrip patch array antenna complex. The invention presents a technique for obtaining a broad bandwidth, low axial-ratio antenna, not a scanning or phased array antenna subsystem. Each patch is sequentially rotated and phased accordingly to produce a nearly circularly polarized radiated field. Also, disclosed is how to design or select the substrate material between the driven and parasitic patch antennas. However, there is no direct relationship between Pett, et al and the present invention. The HRSRA disclosed herein provides a technique and a method for producing phase agile antennas, regardless of the type of polarization.
U.S. Pat. No. 5,334,958, granted Aug. 2, 1994, to R. W. Babbitt, et al., discloses a ferroelectric slab microstrip phase shifter, where a plurality of phase shifters are formed as a single unit. The phase shifting technique of Babbitt, et al., exploits a slab of ferroelectric material upon which the microstrip line is patterned. This same line is then biased and an electric field is generated in the slab perpendicular to the propagation velocity. This is a rather brute force approach to providing a ferroelectric phase shifter. This design does not lend itself to monolithic integration with an array of antenna elements and there is no reference made to any reflectarray implementation.
U.S. Pat. No. 5,210,541, granted May 11, 1993, to P. Hall., et al, discloses microstrip patch arrays for satellite communications using circularly polarized beams. Detailed is an antenna that is capable of an arbitrarily large number of radiating beams. The intended use is for simultaneous or switched coverage of a wide field of view. A technique for producing circularly polarized radiation is also discussed. This antenna design, however, is not intended to provide multiple scanning beams, only fixed multiple beams. Furthermore, no method of inserting phase shifting devices is disclosed, nor is there any reference to any type of ferroelectric phase shifter.
U.S. Pat. No. 5,124,713, granted Jun. 23, 1992, to P. E. Mayes, et al., discloses a planar microstrip thin patch antenna of subarrays for reception of circularly polarized signals. This invention relates to a bandwidth improved circularly polarized patch antenna. The bandwidth improvement over prior art is obtained by using multiple slots at strategic locations in the ground plane to couple to microstripline in order to excite multiple modes in proper phase relationships. While such an antenna could certainly be used in a phased array application this invention bears no other relationship to the ferroelectric phase shifter based reflectarray disclosed, herein.
U.S. Pat. No. 4,853,660, granted Aug. 1, 1989, to Ernst F. R. A. Schloemann, discloses ferromagnetic film dielectric substrate and microwave devices. Detailed is a multilayer ferromagnetic circuit than can be tuned with an appropriate magnetic field. The proposed uses for this structure are tunable bandstop microwave filters and microwave switches. It is conceivable that the structure could be used for phase shifting of a microwave signal. However, Schloemann's invention is based on ferromagnetic effects, as opposed to ferroelectric effects. The basic materials' technology is different, and the method to control the circuitry is entirely different. However, Schloemann does not describe how one would use the device for phased array antenna applications, and makes no mention of reflectarray antennas. Schloemann's circuit structure bears no resemblance to a coupled microstripline configuration.
U.S. Pat. No. 3,906,514, granted Sep. 16, 1975, to H. R. Phelan, discloses an element array for use with a plurality of similar element antennas in an array. The element antenna receives and re-radiates circular polarized electromagnetic energy, such that the re-radiated energy is of the same polarity as the received energy. Further disclosed is a version utilizing a dual polarization spiral element antenna wherein the spiral arms length and spiral diameters are chosen and configured to achieve phase control.
To achieve phase control, Phelan discloses the use of interleaved spiral arms that are connected by diodes. The number of bits of phase shift is limited by the allowable number of arms that are practically interwoven. Hence, the resulting antenna pattern suffers from seriously limited phase quantization. Further, the efficiency of this antenna is limited by impedance mismatches between the antenna elements and the switching devices (i.e., diodes).
Reflectarrays have been theoretically analyzed and several reflectarrays at lower frequencies have been demonstrated. A basic concept, that was based on the use of spiral elements, was introduced H. R. Phelan in 1975. Later, a fixed beam microstrip patch reflectarray has been demonstrated, but did not provide scanning capabilities.
The present invention differs from the aforementioned prior art in that the approach is not limited by the insertion loss or power handling capability of the switching diodes. More importantly, it offers a continuously variable phase shift capability, which results in a much higher scan resolution. As previously discussed, the prior art is limited to approximately two bits of phase shift, whereas the design described herein provides phase shifting capabilities of an arbitrary resolution. Consequently, the antenna pattern provides full hemispherical coverage as opposed to a finite number of discrete beams that is characteristic of the technology currently in use.
The newly designed reflectarray scanning antenna utilizes a space-fed approach and integrates phase shifters on the same surface as the antenna elements. It has been demonstrated in the reflectarray antenna that for the coupled line phase shifters and at high frequencies, the patterned conducting layer dominates the microwave losses.
Accordingly, it is therefore an object of the present invention to provide a high resolution scanning reflectarray antenna that provides continuously variable phase shifting capability as opposed to a discrete phase shifting mode of operation.
It is another object of the present invention to provide a high resolution scanning reflectarray antenna that utilizes coupled line structures layered upon thin ferroelectric films to realize a phase shifting element.
It is still another object of the present invention to provide a high resolution scanning reflectarray antenna that uses microstrip patch radiators in lieu of spiral elements.
It is still yet another object of the present invention to provide a high resolution scanning reflectarray antenna that captures the most desirable attributes of the parabolic reflector and direct radiating phased array.
It is another object of the present invention to provide a high resolution scanning reflectarray antenna that provides full beam steering at a reduced or minimal manufacturing cost.
An additional object of the present invention is to provide a high resolution scanning reflectarray antenna that is also conformal with the exception of the feed horn.
Still, an additional object of the present invention is to provide a high resolution scanning reflectarray antenna that provides improved antenna efficiency through a reduction in the power loss per element.
It is a final object of the present invention to provide a high resolution scanning reflectarray antenna that provides a simpler biasing scheme.
These as well as other objects and advantages of the present invention will be better appreciated and understood upon reading the following detailed description of the presently preferred embodiment taken in conjunction with the accompanying drawings.